This invention relates to a process for the anaerobic biological purification of wastewater containing organic pollutants wherein the wastewater to be purified is conducted through two reactors connected in series and operated under anaerobic conditions. The invention also relates to the apparatus for conducting the process.
In conventional wastewater purification processes of this type it is often the case that high-molecular weight, undissolved pollutants present in a wastewater of complex composition are converted, in the first reactor, into dissolved fragments by means of hydrolysis, and these fragments are then reacted to short-chain organic acids, acetic acid, alcohols, H.sub.2, and CO.sub.2 together with the pollutants initially contained in the wastewater in the dissolved state. This is done in an acidification phase by the use of various microorganisms, described as nonmethanogenic and consisting of facultative and obligate groups of anaerobic bacteria. Collectively, these microorganisms are sometimes also identified in the literature as "acid formers".
The resultant organic acids and alcohols from the first reactor are then first converted primarily to acetic acid in an acetogenic phase, i.e., in a phase promoting the formation of acetic acid, in the second reactor. Subsequently, i.e., in a phase favorable to methane production, methane is then produced primarily from the formed acetic acid and from H.sub.2 and CO.sub.2.
This association of the two conversion phases, as discussed above, within the two reactors results from the fact that initially the hydrolysis and acidification reactions generally proceed in parallel and do not depend on each other with the hydrolysis primarily occurring in the first reactor along with acidification and some methane formation. On the other hand the acetogenic and acid forming bacteria and methanogenic bacteria are dependent on each other in their metabolism and therefore, depend on occupying joint living space, and must be in a state of dynamic equilibrium in the second reactor. For a more complete discussion of this process microbiology see the test "Wastewater Engineering, Treatment/Disposal/Reuse", 2nd Edition, Metcalf and Eddy, McGraw-Hill, 1979, pp. 457-458.
In this application it is noted that "difficult to degrade", "gradually degradable", "slowly degradable", "low-rate" and "high molecular weight and undissolved" ingredients are used interchangeably and mean the same thing. Likewise, "rapidly degradable", "rapid rate", "readily degradable and dissolved" and "low molecular weight" ingredients are also used interchangeably and mean the same thing.
In general, by low-rate substances, examples thereof include, but are not limited to, partially dissolved and partially macromolecular materials, e.g., proteins, long-chain fatty acids, fats, vegetable oils, tallow, bacterial and yeast cell-walls, celluloses, hemicelluloses, starch, in emulsified, suspended or colloidal state as discharged, e.g., from slaughterhouses, dairies, rendering plants, oil mills, pharmaceutical and organochemical plants, pulp and paper factories.
Furthermore, the low-rate substances are typified by a rate of metabolism which is significantly lower than rapid-rate substances. For example, acetic acid, as contained in condensates of sulfite pulping plants or glucose, as contained in sugar factory wastewaters, are rapid-rate substances, for as low-molecular, polar substances they are readily dissolved in water, and the metabolic pathway to methane is short. Proteins, fats, vegetable oils, etc., as contained in food producing plants, are on the other hand low-rate substances as they are of high molecular weight and/or are relatively nonpolar they are in a suspended, emulsified or colloidal state; and their metabolic pathway to methane is longer, requiring a hydrolysis and depolymerisation step first.
For a given organic wastewater load, e.g., expressed as COD, low-rate substrates show an overall digestion rate which is typically only 5 to 30% of those found with rapid-rate substances.
The problem in treating such a wastewater stream of complex composition as discussed above containing in addition to readily degradable, low-molecular weight and dissolved ingredients, also high-molecular weight and undissolved ingredients that are difficult to degrade, resides in the fact that the hydrolysis of the difficult-to-degrade wastewater ingredients proceeds slowly, as discussed above, compared to the other components. Thus, this hydrolysis becomes the "rate-limiting" step in the overall process since full degradation can take place only after fermentative bacteria have disintegrated the wastewater ingredients that are difficult to degrade into smaller more easily digestible particles. This necessarily requires large tank volumes, at least in the first reactor, since typically the entire wastewater stream must be retained in the first reactor until the high-molecular weight, undissolved wastewater ingredients have been converted into dissolved fragments and transformed, together with the readily degradable wastewater ingredients, into acetic acid, alcohols, H.sub.2, CO.sub.2, and organic acids, such as butyric acid and propionic acid.